LONDON (AP) - Protecting 2-0 leads from the first leg of their round-of-16 matches, Barcelona and Bayern Munich are heavily favored to advance in the Champions League this week when they play return matches at home.
Yet there's enough evidence to suggest Europe's two heavyweight teams still have plenty of work to do.
Barcelona has lost three of its last six matches in the Spanish league, giving Manchester City renewed hope the two-goal deficit may not be too much to bridge.
Arsenal, meanwhile, heads to Bayern having won 2-0 at the Allianz Arena in a round-of-16 second leg last season.
In this week's other second legs, Atletico Madrid leads AC Milan 1-0 and Paris Saint-Germain is 4-0 in front against Bayer Leverkusen.
Here is a lookahead to the games in Europe's top knockout competition on Tuesday and Wednesday:
BAYERN MUNICH vs. ARSENAL
Bayern is heeding the lesson learned from losing to Arsenal at home in the last 16 last season, when only away goals ensured progress before it went on to win the title. Another 2-0 loss on Tuesday would level the aggregate score.
"If we let Arsenal have the ball, we'll have big, big problems," said Bayern coach Pep Guardiola, who otherwise has little to worry about.
Guardiola had a full squad to choose from for Saturday's 6-1 rout at Wolfsburg. Defender Holger Badstuber was among the 20 players to travel after recovering from a torn cruciate ligament in his right knee.
The win was Bayern's 16th in a row, a record in the Bundesliga, where the side is undefeated in 49 games.
In inspiring Arsenal to a 4-1 win over Everton in the FA Cup quarterfinals on Saturday, Germany playmaker Mesut Ozil returned to the kind of form that persuaded the Gunners to break their club record to sign him from Real Madrid in the summer.
ATLETICO MADRID vs. AC MILAN
Atletico Madrid has two reasons to feel good about its chances to advance past AC Milan on Tuesday.
First, leading scorer Diego Costa will be well rested after missing Saturday's Spanish league win at Celta Vigo due to a one-game suspension. Costa scored the winner in Atletico's 1-0 first-leg victory at Milan.
Second, David Villa has found his scoring touch after firing home two well-taken goals with Costa unavailable over the weekend in Atletico's 2-0 victory in Vigo that kept it in the Spanish title chase.
Coach Diego Simeone may opt to return to aligning both strikers as he did earlier in the season after not playing Villa in the first leg in Italy or in a recent derby against Real Madrid.
Milan has lost its last two Serie A matches, but has Mario Balotelli back from a shoulder injury that had sidelined him since the first leg.
BARCELONA vs. MANCHESTER CITY
Barcelona is playing its worst soccer since the end of last season, when the team limped to the Spanish league title.
After apparently being rejuvenated under first-year coach Gerard Martino, the side has dropped points in five of the last nine rounds to go from pacesetter to chasing Real Madrid and Atletico Madrid.
With its domestic title defense in question, failing to protect a 2-0 first-leg advantage over City at the Camp Nou and an early exit from Europe would be nothing less than disastrous for the Catalan club.
The match comes four days after Barcelona lost 1-0 at relegation-threatened Valladolid, when only Lionel Messi appeared up to the task.
Barcelona midfielder Sergio Busquets said his team needs to rediscover "the desire to win every game."
"We have had enough after this loss," Busquets said.
City, meanwhile, will be hoping to build on its winning momentum since lifting the League Cup. It will have forward Sergio Aguero back from injury.
PARIS SAINT-GERMAIN vs. BAYER LEVERKUSEN
PSG is all but qualified for the quarterfinals and this gives coach Laurent Blanc the chance to give some of his key players a breather for the return leg against Leverkusen.
Only a monumental collapse will see the German side through. With Zlatan Ibrahimovic in a hungry mood and PSG scoring freely, it seems another hiding is the more likely scenario for Leverkusen, which has lost five of its past six games.
With a tough match to follow on Sunday against in-form Saint-Etienne, it seems Blanc is already preparing ahead.
"We have an important game against Saint-Etienne coming up," he said. "Whatever team is selected, there's always the same desire to win. It's important for the big teams to have this."
Although Ibrahimovic should start, he could come off in the second half, giving Blanc the chance to switch Edinson Cavani from the right wing and play him in his favored center-forward role.